But a study by Britain's Food and Environment Agency with the University of Exeter says the results of the original research were flawed.

The study, published in the journal Science, does not deny that pesticides could be harmful to individual bees but argues there is no evidence they cause the collapse of whole colonies.

"We do not yet have definitive evidence of the impact of these insecticides on honeybees and we should not be making any decisions on changes to policy on their use," said James Cresswell, the ecotoxicologist who led the latest study.

The previous research, led by French scientist Mikaël Henry and published in Science in April, showed the death rate of bees increased when they drank nectar laced with the neonicotinoid pesticide, thiamethoxam, the active ingredient in Cruiser OSR.

Henry's work calculated this would cause a bee colony to collapse completely but Cresswell said the French study seems to have used an inappropriately low birth rate, underestimating the rate at which colonies can recover from the loss of bees.

"They modelled a colony that isn't increasing in size and what we know is that in springtime when oilseed rape is blossoming they increase rapidly," Cresswell told Reuters.

The French study has been cited by scientists, environmentalists and policy-makers as evidence of the impact of these pesticides on bees, which are declining around the world.

"We know that neonicotinoids affect honeybees, but there is no evidence that they could cause colony collapse," said Cresswell. "When we repeated the previous calculation with a realistic birth rate, the risk of colony collapse under pesticide exposure disappeared."

Cresswell said Henry's research also used a dosage of pesticide equivalent to a whole day's intake by the bees, akin to testing the effect of coffee on people by making them drink eight cups in one go, rather than spread out over the day.

Henry was not immediately available for comment.

Neonicotinoids are among the most widely-used agricultural insecticides.

"I am definitely not saying that pesticides are harmless to honeybees, but I think everyone wants to make decisions based on sound evidence, and our research shows that the effects of thiamethoxam are not as severe as first thought," the British scientist said.